While electric power steering devices are coming into general use, there are growing demands for a thinner nut and a simple circulation structure thereof. As a trend, a deflector-type ball screw becomes mainstream. In the deflector-type ball screw, a deflector (also called “piece”) is embedded in the nut to circulate balls. In the deflector, there is formed a return groove that connects both ends of a loaded ball rolling groove of less than one turn. Then, the return groove of the deflector acts to make each ball, which is rolling in the loaded ball rolling groove spirally formed around the screw shaft, go over a thread of the screw shaft before the ball revolves completely around the screw shaft and return back to a starting point of the loaded ball rolling groove.
For the purpose of reducing possible damage of the deflector and manufacturing costs, there has been devised a ball screw having less component count. For example, the patent document 1 discloses a ball screw having a loaded ball rolling groove and a ball return groove integrally formed in a semicircle-shaped nut by molding. According to the invention disclosed in the patent document 1, the loaded ball rolling groove and the ball return groove are formed at the same time when the nut is half-rounded.
In addition, the patent document 2 discloses “a ball screw, which is easy to form and assemble, comprising a screw shaft having an outer surface with a spiral groove formed thereon, a ball nut having an inner surface with a spiral groove formed thereon facing the spiral groove of the screw shaft and a plurality of balls arranged rollably between the spiral groove of the ball nut and the spiral groove of the screw shaft, characterized in that the ball nut has an inner circulation path (corresponding to a return groove) which is connected to the spiral groove and consists in a circular ball path inside the ball nut, and the ball nut is axially cut at a part of its circumference with a radially cut area passing through the inner circulation path” (see patent document 2, claims for Utility Model). This patent document 2 also discloses “a tubular member 6 of the ball nut which is axially cut at a part of the circumference and therefore, easily detached from a mold by opening up the cut area 8 even when it is formed by plastic forming with a male mold (patent document 2, page 5, lines 7-11).    Patent document 1: Japanese patent laid-open publication No. 56-46158    Patent document 2: Japanese utility model laid-open publication No. 58-52358